This invention relates to electrostatography and more particularly to an improved method and apparatus for transferring toner images to receiver sheets.
Ideally, in transferring a toner image from an image-bearing member to a receiver sheet, the electrostatic transfer force on the toned image must be made as high as possible in the transfer nip. Increasing the applied electric field increases the electrostatic force. However, the electric field is limited by air breakdown (ionization) which occurs when the electric field exceeds the Paschen limit (see R. M. Schaffert, Electrophotography, Focal Press, New York (1975), pp. 514-518). While a small amount of ionization can be tolerated, excessive ionization causes image defects and reduces transfer efficiency.
Three distinct regions can be identified in the transfer region: pre-nip, in-nip, and post-nip (see FIG. 1). When using a conventional resistive transfer roller to transfer toner to a receiver the electric field in the air gaps and toner stacks increases as the image traverses the pre-nip and in-nip regions of the nip. In the post-nip region the electric field decreases as the image moves away from the nip. In the pre-nip region it is desirable to keep the magnitude of electric field low to prevent i) premature toner transfer across large air gaps, which blurs the image, and ii) pre-nip ionization, which causes image mottle and poor transfer efficiency. The electric field in the transfer nip (the in-nip region), however, must be larger than in the pre-nip region because this is where transfer of the toned image should occur. Ideally, the electric field in the transfer nip is made as large as possible without allowing significant ionization or pre-nip transfer. The electrical properties of the transfer roller must be carefully selected to maximize the electric field used for toner transfer and, at the same time, minimize the amount of ionization.
It is well known in the art to use a resistive transfer roller to optimize toner transfer from an imaging member to a final receiver (paper). Meager, U.S. Pat. No. 3,781,105 (1987) describes the use of a transfer roller for transferring toner images to a receiving sheet. This reference suggests the transfer roller have a blanket with a resistivity of 10.sup.9 to 10.sup.11 ohm-cm.
Bartholmae and Tompkins, U.S. Pat. No. 5,276,490 (1994) and Koike et al. U.S. Pat. No. 5,303,013 (1994) disclose the use of transfer rollers containing multiple parallel electrodes to aid paper handling and also to control the application of an electrical bias during the transfer of toner images.
Zaretsky, U.S. Pat. No. 5,187,526 (1993) points out that transfer can be improved by separately specifying the resistivity of an intermediate transfer roller and a second transfer roller, which form a nip for transfer to paper.
One difficulty encountered by the aforementioned techniques of utilizing transfer rollers is the limitation imposed by air breakdown (ionization) in the vicinity of the nip in which the toner is transferred to the receiver. Air breakdown degrades the transfer efficiency and image quality of toner images, especially multi-color images, by altering the quantity of charge on the toner particles. In practice, this problem is amplified because transfer rollers are typically doped with anti-stats or other conducting materials that are sensitive to fluctuations in temperature and relative humidity. There is a need to overcome these problems in order to improve the transfer to paper, especially for high quality color imaging and it is an object of the invention to provide a method and apparatus for transferring a toner image to a receiver sheet that overcomes or minimizes such problems.